The efficiency of switch mode power converters (SMPC), also referred to as switch mode power supplies (SMPS), may be improved by replacing the diode conventionally used for rectification at an AC to DC output side of the converter by a switch, which is typically implemented as a MOSFET. By appropriately controlling the timing of the opening and closing of the switch, it can provide rectification—that is to say it may be used to block reverse currents, whilst allowing forward current to flow—without the forward voltage drop associated with a conventional rectification diode. Since the timing of such a switch is synchronised to the timing of the current, this is generally referred to as synchronous rectification.
In practice, the efficiency improvement associated with zero rectification losses is not fully achievable due to the voltage drop over the conducting channel of the synchronous rectification MOSFET, its gate charge, and the supply current required for the controller for the synchronous rectification.
Control of a synchronous rectification (or syncrec) MOSFET may be enhanced by regulating the voltage on its gate, such that the MOSFET operates in linear mode, rather than as a simple switch. For example, U.S. Pat. No. 5,946,207 discloses a synchronous rectifier for a flyback converter for improved switching timing and efficiency in which the gate of the FET is controlled from an amplifier which compares the drain-source voltage of the FET with the voltage of a reference voltage source.
A trade-off is normally required in a feedback loop including an amplifier and operating as a regulator, between a fast response time, which requires a relatively high open loop gain from the amplifier, and a stable operation, which requires a relatively low open loop gain from the amplifier.